1. Field of the Invention
The invention relates to an image forming apparatus used in a copier, printer or facsimile machine.
2. Description of Related Art
A conventional image forming apparatus is disclosed in EP 587,366 (JP-A-6-155798), where an aperture electrode is arranged to contact a toner supply device, such as a toner carrying roller. Contact is through a toner layer to provide the aperture electrode with a low voltage.
The EP 587,366 image forming apparatus is constructed as shown in FIG. 11. A toner carrying roller 114 opposes a sheet of paper 120 through an aperture electrode body 101. A back electrode roller 122 is provided on the back of the sheet 120. In the image forming apparatus, as shown in FIG. 12, a plurality of openings or apertures 106, each having a suitable diameter such as approximately 100 .mu.m, are formed in one surface of an insulating sheet 102. The sheet 102 is formed of polyimide with a thickness of approximately 25 .mu.m. The apertures 106 are formed in a row extending lengthwise of the aperture electrode body 101. A control electrode 104, formed of a suitable material such as copper foil, has a thickness of approximately 8 .mu.m and width of approximately 20 .mu.m, and is positioned around each of the apertures 106. The aperture electrode body 101 is arranged with a surface opposed to the paper 120. A control voltage, corresponding to an image signal from a control voltage applying circuit 108, is applied to the control electrode 104. A voltage is applied from a DC source 124 to the back electrode roller 122 to control the jumping of toner 116 from the toner carrying roller 114 to form an image on the paper 120.
A control electric field for the control electrode 104 is generated between the control electrode 104 and the interior of the aperture 106, and between the aperture 106 and a toner carrying surface of the toner carrying roller 114, which is opposed to the aperture 106. Since the control electric field can be directly applied to the toner 116, excellent control can be obtained. Further, some of the toner 116 receives a mechanical or similar motive force resulting from the contact with the aperture electrode body 101. Even if the toner 116 is moved into an aperture 106 corresponding to a non-image portion, the toner 116 is controlled by the electric field so it does not pass through the aperture 106. Thus, excellent toner control can be obtained. It is therefore possible to obtain a high quality image. Since the toner carrying roller 114 is opposed to the aperture electrode body 101 and separated by a toner layer, the apertures can be positioned relatively close to each other, and a flow of toner can be accurately controlled, even with a low control voltage. Accordingly, the device is inexpensive with high reliability, without requiring expensive controlling drive elements. However, on the other hand, the following problems are evident in the conventional image forming apparatus.
In the conventional image forming apparatus, the aperture electrode body is in contact with the toner layer carried by the carrying roller. The toner particles in contact therewith are rolled on the carrying roller, whereby an adhesive force of the toner particles on the carrying roller is reduced. Thus, the toner particles are easily separated from the carrying roller making it possible to smoothly control the flow toner passing through the aperture. However, to effectively control the flow by a low voltage drive, it is advantageous that the aperture electrode body thickness be reduced.
However, when the aperture electrode body thickness is reduced, creases or deformation may occur in the vicinity of the aperture. Accordingly, it is difficult to maintain a uniform contact between the aperture electrode body and the toner carrying roller. Thus, a stabilized flow of toner is not possible, and it is difficult to obtain an image of high quality, with even printing.
An arrangement with an aperture electrode body having a reduced thickness is known. A moderate tension can be applied to the aperture electrode body, which is pressed against a toner carrying roller. However, with a smaller apparatus, a width or direction perpendicular to the row of the apertures in the aperture electrode body becomes short, making it difficult to mount the aperture electrode body within the apparatus.
For overcoming the above problems, JP-A-7-9692 discloses an apparatus, which is shown in FIG. 13A. A holding member 207 formed of, for example, polyethylene terephthalate (PET) resin or a ceramic material, is affixed to and reinforced by the surface of an insulating sheet 202 of an aperture electrode body 201 on the side provided with a control electrode 204. This does not produce creases or deformation in the vicinity of an aperture 206. Even if the width of the aperture electrode body 201 is small, a moderate tension generates a stable flow of toner.
However, since the aperture electrode body 201 of the image forming apparatus is reinforced by affixing a film, such as a PET film with an appropriate thickness of, for example 50 .mu.m, on one surface of the insulating film, which is formed from a very thin resin film made polyimide, for example about 25 .mu.m, a problem may arise when the films are affixed using a heat sensitive or a hot melt adhesive, such as a pressure sensitive hot melt adhesive. The aperture electrode body may become warped so as to be greatly curved or rolled in on the PET side, due to different thermal expansion coefficients. Even if the aperture electrode body is mounted in a warp-corrected state, a partial strain remains. Therefore, it becomes difficult to define contact with a toner carrying roller, i.e. a charged particle carrier and it is difficult to obtain an excellent image quality.
Further, as shown in FIG. 13B, the JP-A-7-9692 holding member 207 is formed also on the control electrode 204. In the image forming apparatus, a control voltage corresponding to an image signal is applied to the control electrode 204 from a control voltage applying circuit 108. A jumping voltage is applied from the DC power source 124 to the back electrode roller 122 for controlling the jumping of the toner 116 carried by the toner carrying roller 114. Thus an image can be formed on the paper 120. A holding member 207 on the control electrode 204 can affect an electric field generated between the control electrode 204 and the back electrode roller 122, to adversely affect a toner flow control by the aperture electrode body 201.
Further, the aperture electrode body 201 is constructed by forming an aperture 206 after the holding member 207 is joined to the insulating sheet 202, which is provided with the control electrode 204. However, it is very difficult to bore the aperture 206 at a suitable position in the center of the control electrode 204 with the control electrode 204 interposed between the insulating sheet 202 and the holding member 207. There was a very high possibility that the control electrode 204 will be scratched and consequently cause deteriorated printing quality when an aperture 206 is bored.
Moreover, since copper wiring is provided on only one surface of the aperture electrode body 201 for leads, the copper wiring becomes rounded or greatly bent when etching to form the wiring. When a warp, deformation or creases occur in the aperture electrode, it is difficult for the aperture electrode body 201 to contact the toner carrying roller 122. This leads to a disturbance, such as lateral irregularity of an output age, and greatly affects printing.
Further, when the deformation or creases occur in the aperture electrode body 201, it is difficult to process apertures in the aperture electrode body 201 or mount the drive IC. That is, the accuracy of locating apertures becomes poor or the mounting of the drive IC becomes difficult, due to the deformed creases.